THE RHIZOCTONIA DISEASE OF POTATOES. 205 



beginning of late blight or Phytophthora decay. This brown- 

 ing varied from a circle around the stem a centimeter or less in 

 diameter to a large, discolored area covering the entire basal 

 end of the tuber. The lesions were dry in nature, and in the 

 more advanced stages brownish black in color, and more or less 

 covered with Rhizoctonia filaments. The fungus appeared to 

 be entirely superficial. As far as could be judged by micro- 

 scopical examinations it did not penetrate beneath the skin to 

 any extent. Occasionally discolored streaks following the line 

 of the vascular bundles penetrated the flesh for a short dis- 

 tance but not trace of fungus mycelium could be found in them 



A much more serious or advanced form of the injury of the 

 stem end of the tuber was of frequent occurrence. On such 

 cases after the branch stem is killed back to its junction with 

 the tuber there begins a progressive degeneration or necrosis 

 of the tissues of the latter from this point in all directions, 

 forming deep cavities, sometimes two or three centimeters in 

 diameter and extending a like distance into the flesh of the 

 potato. This is well shown in Fig. 70. Here again the fungus 

 does not penetrate deeply into the tissues. The interior of the 

 cavity is lined with a dark, brown colored material which 

 appears to be made up of interwoven masses of Rhizoctonia 

 filaments and the remains of dead cell walls which show more 

 or less corky developments, mixed with large quantities of free 

 starch, which latter appears to be entirely unchanged. This 

 form of the disease need not be confused with the decay of the 

 stem end of the tuber caused by the blackleg organism. 



The fungus appeared to be able to attack tubers independent 

 of infection spreading from the stem. The lesions thus pro- 

 duced may be divided roughly into 3 different groups, two of 

 which, and possibly all 3, are stages of the same thing. One of 

 these is characterized by a pronounced russeting of the skin 

 over areas of varying dimensions, leading into a corky develop- 

 ment somewhat resembling common potato scab. The potato 

 may largely recover from these attacks, the corky layer slough 

 off, or be easily rubbed oft", leaving slight oval depressions cov- 

 ered by healthy skin which had formed beneath the cork. These 

 resemble quite closely the healed wounds on tubers which have 

 been eaten by white grubs. 



